1. Technical Field
This invention generally relates to the field of galvanic cathodic protection of steel embedded in concrete structures, and is particularly concerned with the performance of embedded sacrificial anodes, such as zinc, aluminum, and alloys thereof.
2. Background Art
The problems associated with corrosion-induced deterioration of reinforced concrete structures are now well understood. Steel reinforcement has generally performed well over the years in concrete structures, such as bridges, buildings, parking structures, piers, and wharves, since the alkaline environment of concrete causes the surface of the steel to “passivate” such that it does not corrode. Unfortunately, since concrete is inherently somewhat porous, exposure to salt over a number of years results in the concrete becoming contaminated with chloride ions. Salt is commonly introduced in the form of seawater, set accelerators, or deicing salt.
When the chloride reaches the level of the reinforcing steel, and exceeds a certain threshold level for contamination, it destroys the ability of the concrete to keep the steel in a passive, non-corrosive state. It has been determined that a chloride concentration of 0.6 Kg per cubic meter of concrete is a critical value above which corrosion of the steel can occur. The products of corrosion of the steel occupy 2.5 to 4 times the volume of the original steel, and this expansion exerts a tremendous tensile force on the surrounding concrete. When this tensile force exceeds the tensile strength of the concrete, cracking and delaminations develop. With continued corrosion, freezing and thawing, and traffic pounding, the utility or integrity of the structure is finally compromised and repair or replacement becomes necessary. Reinforced concrete structures continue to deteriorate at an alarming rate. In a recent report to Congress, the Federal Highway Administration reported that, of the nation's 577,000 bridges, 266,000 (39% of the total) were classified as deficient, and that 134,000 (23% of the total) were classified as structurally deficient. Structurally deficient bridges are those that are closed, restricted to light vehicles only, or that require immediate rehabilitation to remain open. The damage on most of these bridges is caused by corrosion. The United States Department of Transportation has estimated that $90.9 billion will be needed to replace or repair the damage on these existing bridges.
Many solutions to this problem have been proposed, including higher quality concrete, improved construction practices, increased concrete cover over the reinforcing steel, specialty concretes, corrosion inhibiting admixtures, surface sealers, and electrochemical techniques, such as cathodic protection and chloride removal. Of these techniques, only cathodic protection is capable of controlling corrosion of reinforcing steel over an extended period of time without complete removal of the salt-contaminated concrete.
Cathodic protection reduces or eliminates corrosion of the steel by making it the cathode of an electrochemical cell. This results in cathodic polarization of the steel, which tends to suppress oxidation reactions (such as corrosion) in favor of reduction reactions (such as oxygen reduction). Cathodic protection was first applied to a reinforced concrete bridge deck in 1973. Since then, understanding and techniques have improved, and today cathodic protection has been applied to over one million square meters of concrete structure worldwide. Anodes, in particular, have been the subject of much attention, and several different types of anodes have evolved for specific circumstances and different types of structures.
The most commonly used type of cathodic protection system is impressed current cathodic protection (ICCP), which is characterized by the use of inert anodes, such as carbon, titanium suboxide, and most commonly, catalyzed titanium. ICCP also requires the use of an auxiliary power supply to cause protective current to flow through the circuit, along with attendant wiring and electrical conduit. This type of cathodic protection has been generally successful, but problems have been reported with reliability and maintenance of the power supply. Problems have also been reported related to the durability of the anode itself, as well as the concrete immediately adjacent to the anode, since one of the products of reaction at an inert anode is acid (H+). Acid attacks the integrity of the cement paste phase within concrete. Finally, the complexity of ICCP systems requires additional monitoring and maintenance, which results in additional operating costs.
A second type of cathodic protection, known as galvanic cathodic protection (GCP), offers certain important advantages over ICCP. GCP uses sacrificial anodes, such as zinc and aluminum, and alloys thereof, which have inherently negative electrochemical potentials. When such anodes are used, protective current flows in the circuit without need for an external power supply since the reactions that occur are thermodynamically favored. GCP therefore requires no rectifier, external wiring or conduit. This simplicity increases reliability and reduces initial cost, as well as costs associated with long term monitoring and maintenance. Also, the use of GCP to protect high-strength prestressed steel from corrosion is considered inherently safe from the standpoint of hydrogen embrittlement. Recognizing these advantages, the Federal Highway Administration issued a Broad Agency Announcement (BAA) in 1992 for the study and development of sacrificial anode technology applied to reinforced and prestressed bridge components. As a result of this announcement and the technology that was developed because of this BAA, interest in GCP has greatly increased over the past few years.
In PCT Published Application WO94/29496 and in U.S. Pat. No. 6,022,469 by Page, a method of galvanic cathodic protection is disclosed wherein a zinc or zinc alloy anode is surrounded by a mortar containing an agent to maintain a high pH in the mortar surrounding the anode. This agent, specifically lithium hydroxide (LiOH), serves to prevent passivation of the zinc anode and maintain the anode in an electrochemically active state. In this method, the zinc anode is electrically attached to the reinforcing steel causing protective current to flow and mitigating subsequent corrosion of the steel.
In U.S. Pat. No. 6,217,742 B1 and in allowed U.S. patent application Ser. No. 08/839,292 filed on Apr. 17, 1997 by Bennett, the use of deliquescent or hygroscopic chemicals, collectively called “humectants” is disclosed to maintain a galvanic sprayed zinc anode in an active state and delivering protective current. In U.S. Pat. No. 6,033,553, two of the most effective such chemicals, namely lithium nitrate and lithium bromide (LiNO3 and LiBr), are disclosed to enhance the performance of sprayed zinc anodes. And in U.S. Pat. No. 6,217,742, issued Apr. 17, 2001, Bennett discloses the use of LiNO3 and LiBr to enhance the performance of embedded discrete anodes. And, finally, in U.S. Pat. No. 6,165,346, issued Dec. 26, 2000, Whitmore broadly claims the use of deliquescent chemicals to enhance the performance of the apparatus disclosed by Page in U.S. Pat. No. 6,022,469.
The addition of synthetic fiber to concrete was originally developed by Soloman Goldfein in the mid 1960's to improve “blast” or impact resistance of concrete. The use of fibrillate (net-shaped) synthetic fiber was subsequently developed by Zonsveld in the late 1960's and early 1970's, and both Goldfein and Zonsveld obtained patents for the use of such fibers to reduce early plastic shrinkage cracking of concrete. The dosage of synthetic fibers to reduce shrinkage cracking ranges from about 0.5 to 1.6 pounds per cubic yard (0.3 to 1.0 kilograms per cubic meter). This constitutes about 0.01 to 0.04 percent of fiber by weight. It is believed that the use of synthetic fiber in amounts greater than about 0.1 percent by weight has not been previously contemplated.
Experimentation has shown that the use of lithium hydroxide as taught by Page is substantially ineffective, the enhancement in performance being only short-lived and producing a relatively low protective current. The Page technology is therefore applicable only for cases where chloride contamination and corrosion rates are small. Further experimentation has demonstrated that certain deliquescent or hygroscopic chemicals (humectants) are far more effective for maintaining the zinc anode in an electrochemically active state and delivering protective current. But the use of some of these humectants resulted in an additional problem. If the zinc anode is maintained in a very active and corroding state, then the corrosion products of the zinc anode will accumulate with time. And because the corrosion product of zinc, zincite (ZnO), occupies nearly two times the volume as the parent zinc, this creates tensile stress that can crack the concrete. The use of some humectants, such as LiOH or LiBr, create an environment in which the zincite is relatively soluble, in which case no stress is generated, at least initially. But the use of other humectants, some of which very effectively enhance the flow of protective current, can result in cracking of the concrete after as little as 0.80 Amp-hour of total charge.